SV40 encodes at least two transforming proteins: the 94K A protein (large T antigen) and the 17K F protein (small T antigen). The A protein regulates the initiation of viral DNA replication and transcription and is required to maintain all aspects of the transformed phenotype in some cell types. The F protein has no known function in productive infection and is required for the expression of a defined subset of the transformed phenotype including anchorage-independent growth and cytoskeletal structure. We plan to study the structure and function of the A and F proteins in vitro and to detect structural or enzymatic interactions between the viral transforming proteins and specific cellular proteins.